Britain is pressing hardest for the union to amend an embargo and allow weapons shipments to forces opposing the regime of President Bashar al-Assad, whose troops have made gains recently. “How long can we go on with people having every weapon that’s ever been devised dropped on them while most of the world denies them the means to defend themselves? That is creating extremism, it is radicalizing people,” William Hague, the British foreign secretary, warned in comments to reporters at the start of the meeting.

Mr. Hague said he would seek “common ground” with his European partners but added that “doing the right thing for Syria” is “more important than whether the E.U. is able to stick together on every detail of this.”

At a news conference before a separate meeting in Brussels on relations between Turkey and the European Union, Ahmet Davutoglu, the Turkish foreign minister, said he supported the lifting of the arms embargo against Syria. “If we do not prevent this through the U.N. Security Council, at least we need to support the right of self-defense,” he said.